Optically active diacyltartaric acids are compounds important as optical resolving agents for producing optically active amines important as raw materials of medicines. Recovering an optically active diacyltartaric acid from a diastereomer salt obtained in an optical resolution step, for recycled use of it, is essential for constructing a resource-saving industrial process. As processes for recovering an optically active diacyltartaric acid by dissociating a diastereomer salt of an optically active amine and the optically active diacyltartaric acid, known are a process wherein a diastereomer salt of (S)-1,2-propanediamine and dibenzoyl-D-tartaric acid is added to 9% hydrochloric acid aqueous solution, to precipitate dibenzoyl-D-tartaric acid which is then recovered by filtration {Japanese Patent No. 2712669 (Example 5)}and a process wherein a diastereomer salt of(S)-1,2-propanediamine and di-p-toluoyl-D-tartaric acid is added to 9% hydrochloric acid aqueous solution, to precipitate di-p-toluoyl-D-tartaric acid which is then recovered by filtration {Japanese Patent No. 2917495 (Example 5)}. However, if these processes are employed as they are, the diacyltartaric acid recovered by solid-liquid separation is likely to be aggregated as a block, and a step of grinding it before recycled use is necessary. Furthermore, if the diacyltartaric acid aggregated as a block is used as it is in an optical resolution step, it takes a long period of time till the salt of an amine and an optically active diacyltartaric acid, essential for optical resolution, is formed. Because of these problems, an optically active diacyl-D-tartaric acid with good properties to allow recycled use cannot be recovered. Moreover, also known is a salt dissociation method wherein the diastereomer salt of (4aR, 8aR)-1-n-propyl-6-oxodecahydroquinoline and di-p-toluoyl-L-tartaric acid is treated by a diluted sodium hydroxide aqueous solution, and (4aR, 8aR)-1-n-propyl-6-oxodecabydro-quinoline is extracted with methylene chloride, while disodium di-p-toluoyl-L-tartrate is left in the water layer {JP6-70063B (Production Example 1)}. However, the document does not describe any process for recovering di-p-toluoyl-L-tartaric acid from disodium di-p-toluoyl-L-tartrate.
It would therefore be helpful to provide a process for industrially recovering an optically active diacyltartaric acid capable of being easily used in recycling, by dissociating a salt of an amine and the optically active diacyltartaric acid. It would also be helpful to provide a process for recovering an optically active diacyltartaric acid with good properties by dissociating a diastereomer salt of an optically active amine and the optically active diacyltartaric acid, obtained by optical resolution. It would further be helpful to provide a method for recycling the obtained optically active diacyltartaric acid into an optical resolution step.